No compression occurs on my jpegs after adding jpeg_quality hook to my functions.php file

WordPress uses that when it resizes images to set the quality, but it will not go back and resize images it’s already processed. If it did, your site would be continuously checking your uploads and grind to a halt

You have these choices:

  • Delete your attachments and reupload them
  • Use a tool such as the regen thumbnails plugin to recreate the images ( will take some time )

These will only affect the image sizes, not the original image that was uploaded ( image size full )